Producing stainless steels in parallel operated vessels

ABSTRACT

A process for producing stainless steels, particularly special steels containing chromium and chromium-nickel, in a smelting arrangement having at least two vessels, for supplying a steel foundry. A charge having mostly iron-containing raw scrap materials and partially carbon-containing alloy carriers is melted in a first vessel. At a temperature of 1460° C., the melt is decarburized by the injection of oxygen so as to reduce the carbon content to less than 0.3%. The melt is heated to a tapping temperature of between 1620° C. to 1720° C. and the carbon content is subsequently reduced to 0.1%. A second charge is melted in a second vessel simultaneously with the decarburizing of the first charge in the first vessel.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to a process for producing stainlesssteels, and particularly for producing special steels containingchromium and chromium-nickel, in a smelting arrangement having at leasttwo vessels for supplying a steel foundry.

2. Description of the Related Art

Usually, an electric furnace of conventional construction is used in theproduction of chromium-containing, and chromium-nickel-containingstainless steels. The electric furnace is constructed as a D.C. or A.C.furnace in which scrap and other iron-containing metallic raw material,e.g., pig iron or DRI (Direct Reduced Iron), are melted together with anadequate amount of alloying elements or alloy carriers. The raw or basematerial which is melted for this purpose is tapped off into a ladle ata temperature of 1670° C. to 1700° C. The ladle is subsequently emptiedinto a converter wherein the melt, which contains approximately 2.5%carbon and approximately 1% silicon, is first oxidized or refined byoxygen. The carbon content is next reduced by mixtures of oxygen andnitrogen, and later by mixtures of oxygen and argon.

Depending on the application of different process techniques,decarburization is carried out to produce a final carbon content of lessthan 0.1%. Resulting chromium losses in the slag must then be recoveredby reduction with ferrosilicon or secondary aluminum.

Further, it is known in a three-step process technique to tap off themetal from the converter at carbon contents of approximately 0.2% to0.3% and subsequently to bring the metal to the final carbon content ina separate vacuum oxidation process.

A disadvantage that the previously known methods have in common is thatdecanting or reladling the melt one or more times results in hightemperature losses. These losses must be compensated. For by using ahigh tapping temperature resulting in a high amount of energyconsumption in the primary melting vessel, such as the electric arcfurnace. In addition to the high amount of energy consumption, the knownmethods disadvantageously cause increased electrode and refractory wearin the electric furnace. Furthermore installation of the converterrequired for the second process step requires substantially highconstruction heights for a surrounding building in order to accommodatea blowing lance and exhaust gas system.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a process for theproduction of stainless steels having fewer process steps and lowerenergy consumption in the individual process steps than in the knownart. A further object is to provide operating equipment that can beconstructed at a lower height.

The process of the present invention begins by melting, in a firstvessel, a first charge having mostly iron-containing raw scrap materialsand partially carbon-containing alloy carriers. At a temperature of1460° C., the melt is decarburized by the injection of oxygen, such asby blowing, so as to reduce the carbon content to less than 0.3%. Nextthe melt is heated to a tapping temperature of between 1620° C. to 1720°C. The carbon content is then reduced to 0.1%. The process of melting asecond charge in a second vessel is accomplished simultaneously with thedecarburizing of the first charge in the first vessel.

The process according to the present invention is carried out in asmelting device having at least two vessels being operated in parallel.Either electrodes for melting the charge, or blowing lances fortop-blowing and/or blowing in oxygen and oxygen mixtures can be used.The vessels thus serve initially as smelting units and then as refiningunits. This has the advantage that the melt can be processed and broughtto a desired temperature without experiencing temperature losses causedby decanting. Scrap, ferronickel, nickel, ferrochromium and othermetallic iron-containing raw materials are melted in each of the vesselsat different times, preferably by electrical energy. This results in abase metal containing mostly iron and having a chromium and nickelcontent close to the final analysis of the steel quality to be produced,particularly as austenites, ferrites and martensites.

In an advantageous embodiment of the present invention, when using highcarbon-containing and/or high silicon-containing ferrochromium, oxygenis blown in by a lance so that the silicon content is reduced. After amelt temperature of 1500° C. to 1600° C. is reached, in the same vessel,the electrode arm is swiveled out. An oxygen lance is swiveled in, whichtogether with nozzles located in a bottom in a side wall of the vessel,oxidizes the melt with oxygen. Of course, mixtures of oxygen andnitrogen, argon, hydrocarbon, and steam can also be used to oxidize themelt. For an average blowing period of 20 to 40 minutes, and at anoxygen injection rate of 0.1 to 2.0 Nm³/t×min., for the oxygen lance andthe injection nozzles, the melt is decarburized to a final carboncontent of 0.10% to 0.015%.

The amount of heat liberated by the blowing process can be utilized toadd coolant, as for example, Ni, FeNi, ferrochromium, scrap and otheriron-containing metallic raw materials such as pig iron masses, DRI oralloying agents, in order to adjust the target analysis and targettemperature.

After blowing, the slag is reduced by reducing agents such asferrosilicon, aluminum or secondary aluminum with the addition of slagdevelopers such as lime and fluorspar for recovering oxidized chromium.The produced steel and/or the slag are/is tapped off. The vessel isagain filled with scrap and alloy carriers, the electrodes are swiveledin, and the scrap and alloy carriers are melted by the electrodes.

The smelting process can and the subsequent blowing process runsuccessively in each of the respective vessels of synchronously betweenthe vessels. After 80 to 120 minutes, a melt can be prepared from onevessel, or in the case of synchronous production of both vessels, a meltis prepared every 40 to 60 minutes for further-processing, in acontinuous casting plant.

The simultaneous use of two vessels not only has the advantage, ofcontinuously supplying a continuously casting plant, but is alsoadvantageous with respect to energy. After the smelting in the firstvessel, for example, the-still hot-electrodes drawn out of the firstvessel can then be introduced into the second vessel to begin thesmelting therein. This process reduces energy consumption and electrodeloss.

The blowing process is carried out to the lowest carbon content whichnaturally leads to high stress of the refractory material in the vesselhearth or bottom. Therefore, the blowing process, in an advantageousembodiment of the present invention, is terminated when a carbon contentof 0.2% to 0.4% is reached. In this embodiment, metal and slag areemptied together into a ladle. The slag is removed by decanting and byscraping. The ladle with the metal melt is then transferred into avacuum vessel, as is known, so that by blowing oxygen the metal melt isrefined to a final carbon content.

Utilizing this process, the typically elaborate installation of aconverter for the blowing process is not necessary particularly in thepreferred embodiments of the present invention, so that investment costsfor the process can be decisively reduced. Furthermore, there is noenergy-consuming decanting of the base metal which has been melted byelectrical energy, especially from the transporting ladle into theconverter, in order to refine the carbon content by using oxygen.

For a particularly high degree of oxidation of silicon, anotherpreferred embodiment of the present invention adds that oxygen duringthe smelting of the charge. A door lance is used in this embodiment soas to avoid special construction steps.

An example of the invention is shown in the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a diagram of the individual process steps.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

First, considering process phase 1, when furnace vessel 1 is at step A,there is a small liquid pool 50 in the furnace vessel 1 on which a newlypoured charge 60 is located. The furnace vessel 1 is closed by means ofa cover 30 through which electrodes 40 project into the upper vessel 20of the furnace 1. In step B, the charge 60 is melted by electricalenergy provided by the electrodes 40. In so doing, the liquid bath level50 rises in the lower vessel 10 of the furnace 1. In step C, the charge60 comprising essentially scrap, ferronickel, nickel, ferrochromium andother iron-containing metallic materials, is melted virtually completelyto liquid base metal 50. When using high carbon-containing and/or highsilicon-containing alloying means, oxygen can be blown in by means of afirst lance 70, so that the silicon content is reduced.

In step D, the base material is completely melted, and the melt has atemperature of 1500° C. to 1600° C. In vessel 1, the electrode device 40is removed from the vessel 1 and a second oxygen lance 80 is swiveled infor changing to process phase 2. In the present diagram, process phase 2is shown in furnace vessel 2 because this process phase runs in vessel 2at the same time as process phase 1 runs in furnace vessel 1 (and viceversa).

Now turning to the process phase 2, in step A, the melt is oxidized witha lance 100 and with bottom nozzles 90 or side nozzles, wherein oxygenor an oxygen mixture is used. The bottom nozzles 90 or side nozzles 90are concentric nozzles 90, having an outer tube, an annular clearance,and a central tube. Oxygen, or an oxygen mixture comprising O₂+N₂,O₂+Ar, or O₂+air, is introduced through the central tube. N₂ or Ar orhydrocarbon or steam or a mixture of these gasses is blown in throughthe annular clearance at the same time.

In step B, after the final carbon content is reached at a tappingtemperature of 1620° C. to 1680° C., the melt slag is reduced by areducing agent such as ferrosilicon or aluminum in order to recoveroxidized chromium. The metal and slag are then tapped together.

In step C, the melt can be decarburized to a final carbon content of0.05%, as shown in alternative 1, or, as shown in alternative 2,transferred after separation of the slag to a vacuum installation at acarbon content of 0.2 to 0.4% and brought to the desired final carboncontent therein. A finished steel is then tapped.

In step D, furnace 2 is filled with a new charge of scrap and partiallywith alloy carriers containing carbon, wherein a liquid pool can belocated in the furnace vessel 2.

As was indicated above, the process is then repeated, beginning withStep A.

What is claimed is:
 1. A process for smelting and refining stainlesssteel in a smelting arrangement having electrodes and at least twovessels for supplying a steel foundry, comprising the steps of: a)melting using electrodes, in a first vessel, a first chargesubstantially including at least one of solid and liquid metalliciron-containing raw materials, and partially including carbon-containingalloy carriers, so as to produce a melt covered with slag; b)decarburizing the melt after reaching a temperature of 1460° C., to acarbon content of less than 0.3% by injecting one of oxygen and oxygenmixtures; c) heating the melt to a tapping temperature of 1620° C. to1720° C.; d) subsequently bringing the melt to a final carbon content ofless than 0.1%, steps a)-d) being carried out in the first vessel; ande) melting a second charge in a second vessel during the decarburizingof the first charge in the first vessel by pivoting the electrodes fromthe first vessel to the second vessel, and repeating steps b) through d)in the second vessel whereby all the process steps for smelting andrefining are carried out respectively in each of the vessels so that thevessels function in a parallel, staggered fashion.
 2. The processaccording to claim 1, further comprising reducing substantially all meltslag by a reducing agent, and subsequently tapping the reduced melt slagtogether with the metal, after the final carbon content and tappingtemperature are reached.
 3. The process according to claim 2, whereinthe reducing agent is one of ferrosilicon, silicon and aluminum.
 4. Theprocess according to claim 1, wherein the decarburizing step includesinjecting one of oxygen and oxygen mixtures by top-blowing incombination with at least one of bottom blowing and side blowing.
 5. Theprocess according to claim 1, wherein the decarburizing step includesdecarburizing the melt to a final carbon content of up to 0.05%, for anoxygen injection period of 20 to 40 minutes.
 6. The process according toclaim 1, further comprising, terminating the oxygen injecting at acarbon content of approximately 0.2% to 0.3% and a temperature ofapproximately 1650° C., reducing slag with one of ferrosilicon andaluminum, emptying the metal and the slag together into a ladle,removing the slag by decanting and by scraping off, and bringing themetal in the ladle to a desired final carbon content of less than 0.1%.7. The process according to claim 1, wherein step e) includes vacuumdegassing the melt to a desired final carbon content of less than 0.1%.8. The process according to claim 1, wherein the melting steps includemelting the charge by means of electrical energy.
 9. The processaccording to claim 2, further comprising, adding oxygen during themelting step so as to cause oxidation of any amount of silicon that maybe present.
 10. The process according to claim 9, wherein the oxygen isadded through a door lance.